Thoracic aortic aneurysms (TAAs) most commonly develop in the ascending region in an asymptomatic manner. Frequently, the first indication of its presence is rupture that commonly leads to death. TAAs are the life-threatening consequence in patients afflicted with a broad range of genetically determined diseases; one of the most common being Marfan's disease. The only current therapeutic strategy for individuals diagnosed with TAAs is surgical options. Consequently, there is a pressing need for mechanistic insight into TAAs to develop effective therapeutics. We have recently demonstrated that AngII infusion also leads to TAAs that are localized to the ascending aorta. AngII-induced TAAs are attenuated by CC Chemokine receptor 2 (CCR2) deficiency. This infers a role for monocyte chemoattractant protein-1 (MCP-1) in TAAs, although CCR2 also interacts with other chemokines. We have recently demonstrated that endothelial-specific deficiency of AT1a receptors imparts a similar degree of attenuation of AngII-induced TAAs as whole body CCR2 deficiency. These two observations could potentially be associated via a paracrine mechanism by which AngII stimulates MCP-1 release, the ligand that determines the major effects of CCR2 stimulation. The AngII-induced TAAs are characterized by profound medial macrophage accumulation that is predominantly on the adventitial aspect. On the basis of this briefly described background, we are proposing to test the central hypothesis that the MCP-1-CCR2 axis promotes AngII-induced TAAs localized to the ascending region through an endothelial-mediated mechanism of macrophage recruitment from the adventitia via aortic region- specific effects. To test this hypothesis, the following specific aims will be addressed: Aim 1. Determine the role of the MCP-1-CCR2 axis in development of AngII-induced TAAs. A. Does whole body deficiency of either MCP-1 or CCR2 promote equivalent reductions in AngII-induced TAAs that are persistent and associated with reduced medial macrophage accumulation? B. Is the source of MCP-1 in promoting TAAs due to AngII releasing this chemokine directly from endothelial or indirectly via specific SMC populations? Aim 2. Determine the origin of medial macrophages accumulating in ascending aortas during development of AngII-induced TAAs. A. What is the sequence of leukocytic infiltration in the development of AngII-induced TAAs and does this correlate to the expression of MCP-1 and CCR2? B. Are macrophages that accumulate in media and adventitia of ascending aortas during AngII infusion derived from blood or tissue origin? C. Is leukocyte accumulation in human ascending aortic aneurysmal tissue associated with MCP-1 and CCR2 expression?